Neverwhere (Illustrated Edition). Neil Gaiman

Richard Mayhew is an unassuming young businessman living in London, with a dull job and a pretty but demanding fiancée. Then one night he stumbles across a girl bleeding on the sidewalk. He stops to help her and the life he knows vanishes like smoke.

Several hours later, the girl is gone, too. And by the following morning Richard Mayhew has been erased from his world. His bank cards no longer work, taxi drivers won't stop for him, his landlord rents out his apartment to strangers. He has become invisible, and inexplicably consigned to a London of shadows and darkness—to a city full of monsters and saints, murderers and angels, knights in armor and pale girls in black velvet. It's a netherworld that exists entirely in a labyrinth of sewer canals and abandoned subway stations. He has fallen through the cracks of reality and has landed somewhere different, somewhere that is Neverwhere.